The importance of predation in benthic stream communities

Abstract: There has been considerable debate whether or not predators has any significant effects on prey densities in streams. This thesis examines the importance of predation in stream communities. I have conducted both field and laboratory studies where I have studied how both vertebrate and invertebrate predators affects benthic prey. Generally, predators have been found to have strong negative effects on several prey taxas densities, as well as predators modifies the behaviour of several invertebrates. Depending on whether a particular predator is present or whether there are several predator types present, effects on benthic prey can be very variable. Due to differences in size, morphology, behaviour and diet among predators, their impact on prey differ as well as prey responses differ depending on predator type. If, for example drift-feeding fish are the dominant predators, their impact may be lower, relative to benthic feeders due to their reliance on alternative prey (terrestrial animals). Predatory invertebrates (leeches, Erpobdella octoculata) are also important components in stream communities. Even if the diet of predatory invertebrates differ to some extent from the diets of fish predators, there is an overlap in prey selection, and predatory invertebrates may consume similar amounts of prey as fish. Multi-predator interactions may also produce non-intuitive results where the presence of several predators not necessarily increase predation pressure, it may actually decrease the risk of being consumed. My work also suggests that the chemical environment in streams is more complex than previous thought. Backflows, localised cues, and habitat heterogeneity can provide detailed information of not only upstream predators, but also about downstream predators. I have also found several examples of trophic cascades in my field studies. Bullheads (Cottus gobio) Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) have positive effects on periphyton through their negative effect on grazers. The amount of terrestrial animals in predators diet has also been found to indirectly affect the strength of trophic cascades. When trout consumes large amount of terrestrial animals their impact on lower trophic levels decreases.